In a gas turbine having a compressor, a combustor, and a turbine, fuel gas and air compressed by the compressor are combusted in the combustor, the generated combustion gas is supplied to the turbine, and the turbine is rotated. In order to increase the output or efficiency, it is desirable to operate the gas turbine with the temperature of the combustion gas at a turbine inlet increased as much as possible within an allowable range. In order to do so, there is a known method for controlling the flow rate of the fuel gas supplied to the combustor such that the combustion gas temperature becomes equal to a predetermined value using a relationship between the temperature of exhaust gas expelled from the turbine and the pressure of air expelled from the compressor in a case where the combustion gas temperature at the turbine inlet is constant (hereinafter, this control system is referred to as “temperature adjustment control”). Specifically, the temperature of the combustion gas at the turbine inlet decreases from the turbine inlet temperature to the exhaust gas temperature due to adiabatic expansion, which is caused by passing through the turbine, and mixing with cooling air from the turbine vanes or the like. That is, there are five main parameters which determine the exhaust gas temperature when a certain turbine inlet temperature is given, which are: the turbine expansion ratio which determines the temperature decrease amount of the combustion gas due to the adiabatic expansion, the turbine efficiency, the specific heat ratio of the combustion gas, and the volume of cooling air and the temperature of the cooling air which determine the temperature decrease amount due to mixing with the cooling air. When the four parameters of the turbine efficiency, the specific heat ratio of the combustion gas, the volume of the cooling air, and the temperature of the cooling air are assumed not to greatly change according to the operation state of the gas turbine, the exhaust gas temperature is uniquely determined at a certain turbine inlet temperature and a certain turbine expansion ratio. Conversely, it is possible to estimate the turbine inlet temperature from the turbine expansion ratio and the exhaust gas temperature. In the above-mentioned temperature adjustment control, the combustion gas temperature is made equal to a predetermined value by controlling the fuel flow rate such that the measured turbine expansion ratio and exhaust gas temperature match a function of a given turbine expansion ratio and exhaust gas temperature. Here, in the temperature adjustment control in practice, the above-mentioned compressor exhaust air pressure is generally used in place of the turbine expansion ratio given that the inlet pressure of the turbine is equal to the compressor exhaust air pressure apart from the combustor pressure loss and that the outlet pressure of the turbine is equal to the atmospheric pressure (approximately 1 atm) apart from the exhaust pressure loss.
However, particularly in gas turbines which use blast furnace gas or coal gas as fuel, there are cases where the composition of the fuel gas changes greatly. In this case, it is desirable to correct the function of the turbine expansion ratio and the exhaust gas temperature in the temperature adjustment control in consideration of the composition of the fuel gas. For example, Patent Document 1 discloses a gas turbine combustion temperature control method which controls combustion temperature by measuring the discharged air pressure of a gas turbine compressor and a gas turbine exhaust gas temperature, and controlling a gas turbine fuel flow rate on the basis of these measured values. This method includes detecting a calorific value of the gas turbine fuel, calculating changes in the exhaust gas temperature characteristics with respect to the discharged air pressure of the gas turbine compressor using the detected value of the calorific value, correcting the exhaust gas temperature characteristics using the calculated value, comparing the corrected value and an actual measured value of the exhaust gas temperature, and adjusting the fuel flow rate such that the difference in the comparison is minimized.